Today there are many different applications where diffusion is used to enrich a fluid flow with solutes from another fluid flow, or to remove unwanted solutes or substances from the fluid flow. One example is in HVAC (Heating, Ventilation and Air Conditioning) where water vapour can be removed from a gas stream in order to reduce power consumption by reduced condensation in a cooler unit or to recycle energy from exhaust air in e.g. a building. Another example is reverse osmosis for desalinating water.
Different methods are used when it comes to separating water vapour from a fluid; such as rotating wheels with moisture capture or plate heat exchangers with semi permeable membranes. In gas drying technologies bundles of tubing, made of materials like Nafion™ are used.
However, these different methods of removing water vapour from fluids do have certain disadvantages; rotating exchangers are provided with moving parts which cause extra costs for maintenance. Further, rotating exchangers increases the risk of contamination between airstreams. Plate exchangers show low efficiency in regards to enthalpy and Nafion™ tubing is expensive.
Producers of these technologies all try to find the most cost efficient way of producing these effects, and therefore different methods are developed. In conventional plate-based heat- or moisture exchangers, the layers of the exchanger are often made up with spacers or distancing members or a support structure, onto which a membrane is laid. Such structures are common but fail to achieve high cost efficiencies due to their need for spacers, which can become expensive depending on the material used.
Further, the spacers also raise the total weight of the exchanger. Due to the weight, more supports are needed when mounted, and increased weight also increases risks due to handling during maintenance. Also the costs for transportation increase with heavy weight.
In some gas drying technologies a multitude of small tubes are used in order to provide a high moisture exchange surface area coupled with good flow characteristics through the bundles of tubing, while the gas flow characteristics on the outside of the bundle are largely neglected, often without adequate spacing for flow between the tubes.
Tubes in a bundle are usually used in conjunction with another fluid stream that goes in counter- or cross-current to the tubes, but on the outside, between the many tubes.
When using individually made tubes of very small diameter, production cost will become high since small tubes are technically complicated to manufacture and refine into a product, and, as a consequence, the final product will become expensive. Another drawback is when tubes are packed into a bundle; in current contemporary products, no satisfactory space allowance is provided for the flow characteristics in between the tubes.